1. Field of the Invention
The present invention relates to a photodetection device that converts light to electrical signals and an optical disk device.
2. Description of the Related Art
In recent years, there has been a rapid spread in optical disks that store selected data, such as images or sound, in digital format, and in optical disk devices that store data on and read data from optical disks. Optical disk devices can store data by irradiating optical disks with laser light, and can read data using the light reflected back when laser light is irradiated onto optical disks.
An optical disk device that includes a front monitor that detects irradiated laser light and converts it into electrical signals is disclosed in, for example, International Publication NO. WO2004/081921. The optical disc device controls the strength of the laser light based on the electrical signals converted from the laser light by the front monitor As mentioned above, laser light plays a pivotal role in optical disk devices, and it is therefore desirable for the front monitor to convert the laser light into electrical signals with a high degree of accuracy.
An example of this type of front monitor structure is shown in FIG. 7. In the example shown in FIG. 7, a front monitor 70 includes a photodiode D2 that converts the laser light irradiated onto the optical disk into an electrical current, a converter portion 80 that converts the electrical current converted by the photodiode D2 into a voltage, an amplifier portion 90 that includes an amplifier that amplifies the voltage converted by the converter portion 80 and adjusted by variable resistors 82 and 84, and a switch S2 that can be used to select the output from the amplifier portion 90.
As the variable resistors 82 and 84 cannot be incorporated into the front monitor 70 IC chip, they are provided externally to the front monitor 70. The manufacturer adjusts the resistance value of the variable resistors 82 and 84 to adjust the amount of variation in the voltage (for example, the degree of voltage rise) on the variable resistors 82 and 84, thus allowing the front monitor gain to be set to the desired value. Note that the voltage is converted from laser light by the converter portion 80. Two variable resistors and amplifiers are provided in order to ensure compatibility with various types of optical disk having different laser wavelengths, such as compact disks (CDs) and Digital Versatile Disks (DVDs).